1. Field of the Invention
The present invention relates to an organic electroluminescent device (which may hereinafter be called “organic EL device”), an organometallic complex compound used therefor, and a light emitting apparatus and an illumination apparatus using the device.
2. Background Art
Research and development are being vigorously made on organic electroluminescent devices because they can emit light with a high luminance even by low voltage driving. In general, organic EL devices each has one or more organic layers including a light emitting layer and a pair of electrodes sandwiching them therebetween. For light emission, they utilize energy of an exciton generated as a result of recombination, in the light emitting layer, of electrons injected from a cathode and holes injected from an anode.
Using phosphorescent materials contributes to efficiency improvement of the device. As the phosphorescent materials, iridium complexes and platinum complexes are known. There are reports on platinum complex light emitting materials capable of emitting blue to green light (refer to, for example, Japanese Patent Laid-Open Nos. 2007-19462, 2006-261623, and 2006-93665). The light emitting layer of an organic electroluminescent device using phosphorescence is formed by adding a phosphorescent material to a material (host material) involved in charge transport.
Phosphorescent materials however require a wider energy gap than fluorescent materials in principle. This means that due to an increase in IP and decrease in Ea, an injection barrier between an adjacent layer on the cathode side and LUMO level rises, which usually results in such a problem that drive voltage increases.
An increase in the drive voltage causes a reduction in power efficiency per amount of luminescence. In addition, a large electric field load is applied to the device so that the device inevitably has reduced durability. These problems have not yet been solved even by the use of the organic EL devices described in, for example, Japanese Patent Laid-Open Nos. 2006-261623 and 2006-93665.